This invention relates to glass-to-metal sealing and particularly to metallic second anode buttons sealed into the glass funnels of cathode ray tubes. Still more particularly it relates to second anode buttons having good sealing capability but reduced cost.
Cathode ray tubes require numerous electrical potentials to be applied to the electrodes of the tube. One of these, the second anode, has a very high potential, in the order of 30 kV or more in the case of color picture tubes applied thereto. This potential is conventionally applied through the funnel by a connector hermetically sealed therein. This connector is conventionally called a second anode button. It is a substantially hollow, electrically conductive button having a thermal coefficient of expansion that substantially matches that of the glass into which it is sealed. Prior to insertion the button is oxidized so that an oxide-to-oxide bond is formed with the glass.
The two piece buttons comprise a can portion with an annulus fixed therein and have been made from two pieces of a 42% Ni-6% Cr iron alloy (ASTM F31) and more recently from a 47% Ni-6% Cr iron alloy (known commercially as N485). Other iron-based alloys that have been used for glass-to-metal seals include chromium-free nickel alloys (ASTM F30) and 18 to 26% chromium-iron alloys; however, neither of the latter two is known to have been used for anode buttons.
The thermal expansion-contraction differences between glass and metal generate the mismatch stresses that account for much of the breakage of cathode ray tubes that takes place during heat cycles at fabrication. Examination of volume-temperature diagrams for the typical lead or soda-lime glasses used for the funnels show that they exhibit a non-linear behavior that can be matched to the volume-temperature curves of nickel-iron alloys in the 40 to 50% nickel range. However, due to their nickel content, these alloys are significantly more expensive than nickel-free alloys.
It is, therefore, an object of the invention to obviate the disadvantages of the prior art.
It is another object of the invention to enhance the glass-to-metal sealing of second anode buttons.
It is another object of the invention to reduce the cost of second anode buttons.
These objects are accomplished, in one aspect of the invention, by the provision of a two-piece second anode button for a cathode ray tube that comprises a first portion defined by a cup-shaped can having a seating area and a second portion comprising a planar annulus fixed in the seating area. The first portion comprises a nickel-iron alloy having a composition of 47 wt. % nickel, 6 wt. % chromium and the balance iron and the second portion comprises a ferritic stainless steel alloy having a chromium content of about 10 to about 28% by weight.
This button utilizes the preferred material for consummating the glass-to-metal seal and utilizes the less costly material for the annulus, which is entirely contained within the can. The annulus closely matches the thermal coefficient of expansion of the can and the funnel glass and is less costly than using the same material as the can. The can oxidizes well to form tight, hermetic seals with the funnel glass, thereby enhancing the cathode ray tubes with which it is employed.